The present disclosure relates to a method of controlling a multilevel converter, and more particularly, to a method of controlling a multilevel converter for improving a processing time by adopting a switching scheme for voltage balancing.
A gate switching scheme in consideration of voltage balance is used in a multilevel converter system in the related art. In the voltage balancing in the related art, information inputted during a modulation and capacitor voltage information outputted from every sub-module inside an arm are arranged in order of a voltage level. A sub-module is selected using information processed in this way. Specifically a mechanism is employed, in which a sub-module having the highest voltage or the lowest voltage is selected according to current information of the corresponding arm.
According to the voltage balancing in the related art, it is first determined how many sub-modules output an ON signal, and it is then determined which sub-module in one arm is to be in ON state. At this time, a sorting operation of comparing voltage values for the respective sub-modules with each other and arranging the compared voltage values is performed in order to equally distribute the voltages of the sub-modules. Since the sub-modules in ON state are charged and discharged according to an arm current direction, a sub-module having the minimum voltage is selected to be charged when the arm current value is positive and a sub-module having the maximum voltage is selected to be discharged when the arm current value is negative.
FIG. 1 is a view illustrating the voltage balancing in the related art. That is, the number of sub-modules satisfying the ON condition is determined in operation S10 and the sub-modules are sorted according to voltage values in operation S20. Then, a sign of a current value is determined in operation S30. When it is determined that the sign of the current value is positive (YES) in operation S30, the minimum voltage is selected in operation S40. When it is determined that the sign of the current value is negative (NO) in operation S30, the maximum voltage is selected in operation S50.
According to the related art, the longest time is taken in the operation S20 of sorting the sub-modules according to the voltage values. In particular, for a module based multilevel converter applied to large capacity electric power devices, 150 to 200 or more sub-modules are provided in a single arm. A digital processing operation is divided into total 4 sub-operations including converting voltage values of sub-modules into digital values, arranging the converted sub-module values, selecting a sub-module according to a current direction, and applying the sub-module value as a gate signal of the selected sub-module.
Here, the first and second sub-operations, that is, converting of voltage values into digital values and arranging of the converted digital values require much a lot of time. Furthermore, the required time for these two operations also increases infinitely in proportion to an increase in number of sub-modules.